This application is related to and claims priority from Japanese Patent Applications No. 2000-372816 filed on Dec. 7, 2000 and No. 2000-399707 filed on Dec. 28, 2000, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a rotary electric machine which includes a stacked brush formed by layering plural brush layers each having different specific resistance.
2. Description of Related Art
U.S. Pat. No. 5,712,522 (JP-A-9-49478) discloses a starter where a stacked brush (two-layered brush) is adopted to improve a commutation property. As shown in FIGS. 9A-9C, the stacked brush is formed by layering two kinds of brush layers (low-resistance brush layer 100 and high-resistance brush layer 110) each having different specific resistance. The low-resistance brush layer 100 is disposed at the front end side in the rotational direction of a commutator, and the high-resistance brush layer 110 is disposed at the rear end side in the rotational direction of the same.
However, a commutation property cannot be sufficiently improved sometimes. For example, when the width xe2x80x9caxe2x80x9d of the undercut 130 between the neighboring segments 120 of the commutator is larger than the width xe2x80x9cbxe2x80x9d of the high-resistance brush layer 110 (a greater than b), the rear end of the high-resistance brush layer 110 departs from the present segment 120 before the front end (attachment surface between the low-resistance brush layer 100 and the high-resistance brush layer 110) of the high-resistance brush layer 110 comes in contact with the next segment 120. That is, when the rear end of the high-resistance brush layer 110 departs from the present segment 120, the front end of the high-resistance brush layer 110 does not come in contact with the next segment 120, thereby lessening the effect of the stacked brush.
In a recent starter, a speed reduction ratio tends to be increased for reducing its size. Since the number of times of commutation is increased by increasing the revolution speed of an armature, a commutation property is required to be improved. When the armature current is large, shorting current flowing into the brush becomes large when the brush moves to the next segment. As a result, when the rear end of the brush (rear end of the high-resistance brush layer) departs from the segment, a large spark is generated, thereby reducing motor output and a brush lifetime.
Especially, the revolution speed of the commutator of the starter fluctuates largely, and a current is made to flow into the commutator at a rated value in a short time. Therefore, an undercut portion is liable to clog due to wear powder of the brush, so that the undercut width is set comparatively large. Accordingly, the above trouble is liable to occur more often.
When the width of the high-resistance brush layer 110 is enlarged too much, the entire resistance of the brush is increased, thereby reducing a motor output.
Further, in some recent starters, the brush comes in contact with three segments at a maximum at the same time in consideration of the balance between output and a brush lifetime. When the two-layered brush is adopted in this construction, the entire width of the brush is larger than the summated width of the segment 120 and two undercut portions (insulation portions) 130 as shown in FIGS. 10A-10C. In this case, when the width of the low-resistance brush layer 100 is set larger than the summated width of one segment 120 and one undercut portion 130, the following problem arises.
It is assumed that three segments 120are sequentially arranged in the circumferential direction as a first segment 120A, a second segment 120B and a third segment 120C from the front side to the rear side in the rotational direction of the segments.
When the high-resistance brush layer 110 does not come in contact with the second segment 120B before the front end of the low-resistance brush layer 120 comes in contact with the third segment 120C, the following trouble is caused. That is, when the front end of the low-resistance brush layer 100 comes in contact with the third segment 120C (FIG. 10C), a large shorting current flows between the front and rear segments (second segment 120B and third segment 120C) through the low-resistance brush layer 100. In this case, since the shorting current cannot be made lower even by the two-layered brush, a spark is generated at the front end of the low-resistance brush layer 100, thereby reducing the lifetime of the brush.
The present invention has been made in view of the above problem, and its object is to provide a high-performance rotary electric machine where the effect of a stacked brush can be exhibited and less sparks are generated by improving a commutation property.
In the present invention, rotary electric machine includes a stacked brush, where a low-resistance brush layer is disposed at the front end side in the rotational direction of a commutator and a high-resistance brush layer is disposed at the rear end side in the rotational direction. In the rotary electric machine, a width xe2x80x9caxe2x80x9d of an insulation gap, provided between neighboring segments of the commutator, and a width xe2x80x9cbxe2x80x9d of the high-resistance brush layer are determined to satisfy a less than b. Therefore, when the high-resistance brush layer moves to a next segment, the high-resistance brush layer comes in contact with the segments at front and rear sides of the insulation gap at the same time.
Accordingly, the commutation property is improved by exhibiting the effect of the stacked brush, and an output property can be improved. Further, the number of spark generation is reduced by the high-resistance brush layer, and the brush lifetime is increased.
Preferably, the width are further determined to satisfy b/a less than 4. Therefore, the entire resistance of the brush is restricted from being increased, thereby preventing a motor output from being increased, thereby preventing a motor output from being reduced.
In the rotary electric machine, preferably, a width xe2x80x9ccxe2x80x9d of the segment in a circumferential direction and a width xe2x80x9cdxe2x80x9d of the low-resistance brush layer in the stack direction are determined to satisfy d less than a+c. Therefore, when the low-resistance brush layer comes in contact with the present segment and the next segment at the same time, the high-resistance brush layer comes in contact with the present segment together with the low-resistance brush layer. Thereby the shorting current between the segments (present segment and next segment) through the stacked brush is reduced. Thus, the number of the spark generation at the front end of the low-resistance brush layer can be reduced, and the lifetime of the stacked brush can be restricted from being reduced.
Thus, the commutaion property can be improved by the stacked brush, and the output property can be also improved.